Development of an Electrically Matching Boil-Off Calorimeter for Thermophysical Property Measurement of CO2-Based Zeotropic Mixtures

Supercritical CO₂ Brayton cycles have emerged as a promising technology for high-temperature power systems. To accommodate extreme environments, zeotropic mixtures are being explored for their ability to tailor thermodynamic properties to specific applications. While advanced fluid mixing models exist, there is limited experimental data regarding supercritical CO₂ mixtures that corroborates model data. This work addresses this gap by accurately measuring thermodynamic properties using an electrically matched boil-off calorimeter for the validation of REFPROP mixing models. Moreover, to ensure the accuracy of the calorimeter, two calibration runs were conducted using water and pure CO₂ as the test fluids. With these fluids, the calorimeter measurements were consistently found to be within 5% of the corresponding REFPROP values across the entire range of applied power. Initial results of a CO₂/R134a mixture having a molar ratio of 0.9/0.1 respectively found the difference in the REFPROP calculated power and experimental measurements to be within the total expanded uncertainty of the calorimeter and therefore are considered negligible. Further experimentation with a CO₂/R134a mixture having a molar ratio of 0.5/0.5 respectively is currently being conducted to confirm these findings and establish the model validity across the full range of molar ratios.